Method and apparatus to ensure DLL locking at minimum delay

ABSTRACT

A method and apparatus to ensure DLL locking at a minimum delay is provided. In one embodiment, a DLL circuit includes a phase detector, a counter, a programmable delay line, and a counter control circuit. Upon initialization of the DLL circuit, the counter control circuit is configured to cause the counter to count increment, regardless of the phase relationship between a reference clock signal and the output clock signal. The counter continues incrementing, thereby changing the phase relationship between the reference clock signal and the output clock signal by adjusting the delay of the programmable delay line. This eventually results in a phase lock between the reference clock signal and the output clock signal at a minimum delay. Once the DLL achieves a phase lock between the reference clock signal and the output clock signal, the counter increments or decrements its count in order to maintain or re-acquire a lock.

This application is a continuation of U.S. patent application Ser. No.09/901,794 filed on Jul. 9, 2001, now U.S. Pat. No. 6,504,408.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to timing circuits, and more particularly, todelay locked loop (DLL) circuits.

2. Description of the Related Art

Delay locked loops (DLLs) are commonly used in computer systems andother electronic systems in order to ensure proper timing. A typicaldelay locked loop includes a phase detector, a counter, and aprogrammable delay line. The phase detector may be used to compare thephase between two input clock signals, wherein one of the clock signalsmay be a reference clock signal and the other may be a clock signal fedback from the DLL output. The phase detector may generate an UP or DOWNsignals based on the phase relationship between the input clock signals,which may then be driven to a counter. The counter may then count up ordown, thereby adjusting the delay of the programmable delay line untilthe two input clock signals are in phase.

Since a typical phase detector may compare only the phase differencebetween two input clock signals, it is possible for a DLL to havemultiple lock points. Thus, it is no possible for the DLL to lock at apoint that is N times the input period of a reference clock signal,wherein N is an integer value greater than or equal to one. When thegoal is to generate certain type of signals (such as quadrature signals,i.e. signals with a 90° phase difference), locking at a value of Ngreater than one may be unsuitable. In such cases, the designer of theDLL circuit must ensure that the DLL locks at a value of N=1.

SUMMARY OF THE INVENTION

A method and apparatus to ensure DLL locking at a minimum delay isprovided. In one embodiment, a DLL circuit includes a phase detector, acounter, a programmable delay line, and a counter control circuit. Uponinitialization of the DLL circuit, the counter control circuit may beconfigured to cause the counter to increment, regardless of the phaserelationship between a reference clock signal and the output clocksignal of the DLL circuit. More particularly, the counter controlcircuit may cause the counter to increment, even if the phase detectoroutputs initially indicate that the counter is to decrement. The countermay continue incrementing, thereby changing the phase relationshipbetween the reference clock signal and the output clock signal byadjusting the delay of the programmable delay line, until the phasedetector indicates increment, then subsequently indicates decrement.This may eventually result in a phase lock between the reference clocksignal and the output clock signal at a minimum delay at which a phaselock may be achieved among possible delays at which a lock might beachievable. Then, the counter control circuit may cause the counter inincrement or decrement from the indications provided by the phasedetector. Once the DLL achieves a phase lock between the reference clocksignal and the output clock signal, the counter may then increment ordecrement its count as necessary in order to maintain or re-acquire alock.

In one embodiment, the DLL circuit includes a phase detector, a countercontrol circuit, a counter, and a programmable delay line. The phasedetector may receive a reference clock signal, as well as an outputclock signal from the programmable delay line. The phase detector maydetermine the phase relationship between the reference clock signal andthe output clock signal. In response to determining the phaserelationship, the phase detector may generate signals indicating whetherthe counter is to be incremented or decremented, as well as indicatingwhether the DLL circuit has obtained a lock. The phase detector maydrive these signals to a counter control circuit. In response, thecounter control circuit may be configured to generate signals whichcause the counter to increment or decrement. The counter control circuitmay be configured to cause the counter to increment upon initializationof the DLL circuit, wherein initialization includes setting the value ofthe counter output to zero. A re-initialization of the DLL circuit mayoccur responsive the counter control circuit receiving a reset signal.

The counter may increment or decrement depending upon the signalsreceived from the counter control circuit. The output of the counter maybe driven to the programmable delay line, where the value of the countmay adjust the delay of the reference clock signal and thereby producean output clock signal. The output clock signal may be fed back to thephase detector for a phase comparison with the reference clock signal.

By forcing the counter to increment upon initialization or reset of theDLL circuit, the DLL circuit may be able to obtain a phase lock at aminimum delay of the programmable delay line. The delay at which a phaselock occurs may be at the delay at which the output clock and referenceclock frequencies match. In one embodiment, the DLL circuit isconfigured to obtain a phase lock at N=1, wherein N is an integermultiple of the clock period.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects of the invention will become apparent upon reading thefollowing detailed description and upon reference to the accompanyingdrawings in which:

FIG. 1 is a block diagram of one embodiment of a DLL circuit;

FIG. 2 is schematic diagram of one embodiment of a phase detector;

FIG. 3 is a schematic diagram of one embodiment of a counter controlcircuit;

FIG. 4 is a schematic diagram of one embodiment of a delay element; and

FIG. 5 is a block diagram of one embodiment of a carrier medium.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and description theretoare not intended to limit the invention to the particular formdisclosed, but, on the contrary, the invention is to cover allmodifications, equivalents, and alternatives falling with the spirit andscope of the present invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to FIG. 1, a block diagram of one embodiment of a DLLcircuit is shown. DLL circuit 10 includes phase detector 12, counter 14,programmable delay line 16, and counter control circuit 20. DLL circuit10 is configured to receive a reference clock signal (Reference clock inFIG. 1) and produce an output clock signal (Output clock in FIG. 1). Thereference clock signal in this embodiment is received by both phasedetector 12 and programmable delay line 16. Phase detector 12 is alsoconfigured to receive an output clock signal from programmable delayline 16 via a feedback loop. Phase detector 12 is configured todetermine the phase relationship between the reference no clock signaland the output clock signal. Phase detector 12 may also generate signalsresponsive to determining the phase difference between the referenceclock signal and the output clock signal. In the embodiment shown, phasedetector 12 is configured to generate an UP signal and a LOCK signal.The UP signal, when active, indicates that counter 14 is to count up(increment) based on the phase difference between the reference clocksignal and the output clock signal. An indication that counter 14 is tocount down (decrement) based on the phase difference may be providedwhen the UP signal is in an inactive state and the LOCK signal is in aninactive state. An active state of the LOCK signal indicates that DLLcircuit 10 has obtained a phase lock between the reference clock signaland the output clock signal.

Counter control circuit 20 is arranged between phase detector 12 andcounter 14, and is configured to receive the UP and LOCK signals.Counter control circuit 20 may respond to the UP and LOCK signals bygenerating control signals outL or outH. An active state of the outLsignal may indicate to counter 16 that it is to decrement its count,while an active state of the outH signal may indicate to counter 16 thatit is to increment its count. In one embodiment, incrementing thecounter selects a longer delay in the programmable delay line 16, whiledecrementing the counter selects a shorter delay. Thus, if the outputclock leads the reference clock, counter control circuit 20 may causecounter 14 to increment. If the output clock lags the reference clock,counter control circuit 20 may cause the counter to decrement.

Counter control circuit 20 may also be configured, in response toinitialization or reset of DLL circuit 10, to cause counter 14 toincrement even if phase detector 12 indicates that counter 14 shoulddecrement. Counter control circuit 20 may continue forcing theincrementing of counter 14 until phase detector 12 transitions fromindicating increment (UP) to indicating decrement for the first time.Thereafter, counter control circuit 20 may generate the outL and outHsignals which reflect the outputs of phase detector 12. Moreparticularly, the outL and outH signals may reflect the outputs of phasedetector 12 as follows: the OutH signal may be active in response to theUP signal being active, and otherwise may be inactive; and the OutLsignal may be active in response to the UP signal being inactive and theLOCK signal being inactive, and otherwise may be inactive.

Counter 14 may generally increment or decrement in response to the outHand outL signals. More particularly, the counter 14 may increment inresponse to an active outH signal, decrement in response to an activeoutL signal, and hold steady if neither the outH nor outL signals isactive.

Counter 14 may be one of many different types of synchronous binarycounters. The exact number of outputs of counter 14 may depend upon therequirements of programmable delay line 16. Counter 14 may also receiveone or more signal inputs that cause it to increment or decrement (e.g.outL and outH). The output signals produced by counter 14 may be drivento programmable delay line 16 in order to control the amount of delayexperienced by the reference clock signal. Counter 14 may be a rollovercounter or a saturating counter, as desired.

Both counter 14 and counter control circuit 20 are configured to receivea reset signal. The reset signal may be activated to initialize or resetDLL circuit 10. IN response to activation of the reset signal, counter14 may reset its count to zero, while counter control circuit mayactivate the outH signal, thereby forcing counter 14 to beginincrementing, until the phase detector 12 transitions from indicatingincrement to indicating decrement. The phase detector 12 may indicatedecrement initially, but then eventually indicate increment as the phasedifference between the reference clock and the output clock chances inresponse to the forced increments. Subsequently, the phase detector mayindicate decrement again in response to phase changes. Similarly, if thephase detector 12 indicates increment initially, it may eventuallyindicate decrement as the phase difference changes in response to theforced increments.

Programmable delay line 16 may be configured to provide a delay to thereference clock signal, and thereby produce an output clock signal. Theoutput clock signal may be fed back to phase detector 12, which may thendetermine the phase relationship between the reference clock signal andthe output clock signal. Programmable delay line 16 may be configured toadjust the phase relationship between the clock signals. The amount ofdelay provided, as well as the direction of adjustment, may becontrolled by the inputs received from counter 14. In one embodiment,the amount of delay provided by programmable delay line 16 increases ascounter 14 increments, while decreasing as counter 14 decrements.

Alternate embodiments of DLL circuit 10 are possible and contemplated.For example, the output signals generated by phase detector 12 mayinclude UP and DOWN signals in an alternate embodiment, wherein anactive UP signal indicates incrementing, an active DOWN signal indicatesdecrementing, and a phase lock is indicated when both signals areinactive. In another alternative embodiment, DOWN and LOCK signals maybe generated by phase detector 19, wherein an active DOWN signal with aninactive LOCK signal indicates decrementing, and an inactive DOWN signalalong with an inactive LOCK signal indicates incrementing. An activeLOCK signal indicates LOCK. Furthermore, for the embodiments illustratedin FIGS. 2 and 3, signals may be active high (binary 1) and inactive low(binary 0). Other embodiments may define one or more of the signals asactive low and inactive high, with the logic circuit changedaccordingly.

It is noted that, while one embodiment of the counter control circuit 20causes the counter 14 to increment at reset, other embodiments may causethe counter to decrement at reset, as desired. For example, ifincrementing the counter decreases the delay of the delay line 16 anddecremented increases the delay, then decrementing, at reset may bedesirable (and the counter 14 may be initialized to all binary ones, forexample).

Moving now to FIG. 2, a schematic diagram of one embodiment of phasedetector 12 is shown. Phase detector 12 includes a plurality of delayelements 122, a pair of flip-flops, and logic gates to drive outputsignals. Other embodiments are possible and contemplated. The delayelements may be programmable delay elements. In one embodiment, delayelements 122 are programmable delay elements, but are hardwired to apredetermined value.

Delay elements 122 of the embodiment of phase detector 12 shown in FIG.2 are arranged in two chains, the first of which comprises delayelements 122A, 122B, and 122C. The second chain of delay elementscomprises delay elements 122D, 122E, and 122F. The first chain of delayelements is configured to receive the output clock signal from aprogrammable delay line, such as programmable delay line 16 of FIG. 1.The second chain of delay elements is configured to receive thereference clock signal. The output clock signal, received by delayelement 122A, passes through the delay element to the D-input of bothflip-flops 1 and 2, and to delay element 122B. The output clock signalalso passes from delay element 122B to delay element 122C. Similarly,the reference clock signal passes through delay element 122D and delayelement 122E to the clock input of flip-flop 1 and the clock input offlip-flop 2, respectively. In addition, the reference clock signalpasses through delay element 122E to delay element 122F. The presence ofdelay elements 122B and 122C in the first chain, as well as the presenceof delay element 122F in the second chain is primarily to balance theimpedance experienced by the output clock signal and the reference clocksignal, providing approximately equal delay on each signal at the outputof each corresponding delay element.

As previously noted, the output clock signal passes through delayelement 122A to the input of both flip-flops 1 and 2, which are Dflip-flops in this embodiment. Embodiments of phase detector 12utilizing other types of flip-flops are possible and contemplated.Flip-flops 1 and 2 are clocked by the reference clock outputs of delayelements 122D and 122E, respectively. Because the reference clock signalpassing through delay element 122E encounters additional delay,flip-flop 1 may receive a rising edge at its clock input beforeflip-flop 2. The output clock signal passing through delay element 122Aand the reference clock signal passing through delay element 122D mayencounter substantially the same amount of delay, while the signal fromthe output of delay element 122E may be approximately twice that of thesignal output from delay element 122D. Accordingly, flip-flop 1 maycapture the value of the output clock signal earlier in time thanflip-flop 2.

In the embodiment shown, output logic gates may be used to provide anindication as to whether counter 14 is to increment or decrement, aswell as providing an indication as to whether the DLL circuit 10 hasobtained a lock. The UP signal may be active when the outputs (Q) offlip-flops 1 and 2 are simultaneously high. This is due to the fact thatat the rising edge of the reference clock, flip-flop 1 captures a high(i.e. the captured state of the output clock), and at the rising edge ofa delayed reference clock (from the output of delay element E),flip-flop 2 captures a high. This is an indication that the rising edgeof the output clock signal is occurring before the rising edge of thereference clock signal. Thus, by increasing the delay (by incrementingcounter 14), subsequent edges of the output clock signal may be closerto subsequent edges of the reference clock signal.

The LOCK signal may become active when flip-flop 1 captures a low on therising edge of the reference clock signal and flip-flop 2 captures ahigh on the rising edge of the delayed reference clock signal. This isan indication that the rising edge of the reference clock signal iswithin one delay time (as provided by the delay elements) of the risingedge of the output clock signal.

In other cases, both the LOCK and UP signals may be inactive, indicatingthat the rising edge of the reference clock may be occurring before therising edge of the output clock. When both the LOCK and UP signals areinactive, the rising edge of the reference clock signal and the risingedge of the output clock signal may be brought closer together bydecreasing the delay, which is accomplished by decrementing counter 14.

As mentioned above, alternative embodiments of DLL circuit 10 mayinclude a phase detector 12 which generates UP and DOWN signals, or DOWNand LOCK signals. Logic circuitry may be designed to generate an UPsignal whenever the outputs of flip-flops 1 and 2 are simultaneouslyhigh. Logic circuitry may be designed to generate a DOWN signal wheneverthe output of flip-flop 2 is low. Logic circuitry to generate a LOCKsignal may be designed such that the LOCK signal becomes active wheneverthe output of flip-flop 1 is low at the same time the output offlip-flop 2 is high. The logic circuitry for any of these embodimentsmay be designed such that the signals generated are active high oractive low.

It is noted that, in one embodiment, the delay elements 122A-122Finclude an odd number of inverters between the input (IN) and the output(OUT). Thus, the input inverters between the output clock signal anddelay element 122A and the reference clock and delay element 122B areprovided. In other embodiments, an even number of inverters may beincluded in the delay elements and the input inverters may be deleted.

Turning now to FIG. 3, a schematic diagram of one embodiment of acounter control circuit is shown. Counter control circuit 20 isconfigured to receive the UP and LOCK signals from phase detector 12.Responsive to the combinations of signals received, counter controlcircuit 20 may produce an outL signal or an outH signal. The outLsignal, when active, may cause counter 14 to decrement, while the outHsignal, when active, may cause counter 14 to increment.

The outL signal may be active in response to both the LOCK and UPsignals from phase detector 12 being inactive and if counter controlcircuit 20 is not forcing the outH signal active after a reset. The LOCKsignal being inactive is included in the embodiment of FIG. 3 as theupper input of the NOR gate 5, and the remaining conditions are includedthrough the lower input of the NOR gate 5 (which receives the outHsignal). The outH signal may be active in response to the UP signal fromphase detector being active (upper input of the NOR gate 4).Additionally, the outH signal may be active if the complementary (Q witha vertical bar over it) output of flip-flop 3 is active, even if the UPsignal is inactive. Flip-flop 3 is an edge-triggered D-type flip-flop inthis embodiment. The complementary output of flip-flop 3 may becomeactive following a reset of DLL circuit 10, which includes the assertingof the reset signal shown in the drawing. The assertion of the resetsignal clears the flip-flop 3, and thus the complementary output of theflip-flop 3 is activated (binary 1). The complementary output offlip-flop 3 is the lower input to the NOR gate 4 and thus, if thecomplementary output of flip-flop 3 is active, the outH signal may beactive. In this fashion, the counter 14 may count up, regardless of thephase relationship detected by phase detector 12. During this time, theUP signal may become active. On the first transition from an active toan inactive state on the UP signal, flip-flop 3 receives a rising edgeon its clock input (through the inverter 6) and thus captures the Dinput, V_(dd) (a logic 1 in this embodiment). Thus, the complementaryoutput becomes a logical zero, or inactive. The complementary output offlip-flop 3 may remain inactive until the next assertion of the resetsignal. Thus, following the initial state of counter control circuit 20,where the outH signal is asserted regardless of the state of the signalinputs from phase detector 1, subsequent states of the outH and outLsignals may be determined based on the UP and LOCK signals. It should benoted that, if the LOCK signal is asserted, both the outH and outLsignals may be inactive, since the logic of phase detector 12 preventsthe UP and LOCK signals from being active at the same time.

Alternate embodiments of counter control circuit 20 are possible andcontemplated. The specific embodiment chosen for counter control circuit20 may be based on the output signals generated by phase detector 12.For example, if phase detector 12 generates UP and DOWN signals, thecircuit of FIG. 3 may be reconfigured for such an embodiment by removingone of the inverters on the upper input to NOR gate 5 (and coupling theDOWN signal output from the phase detector 12 in place of the LOCKsignal). In another alternative embodiment, the phase detector 12 may beconfigured to generate DOWN and LOCK signal. In such an embodiment, thecircuit of FIG. 3 may be reconfigured by removing one of the inverterson the upper input to NOR gate 5 (and coupling the DOWN signal outputfrom the phase detector 12 in place of the LOCK signal), and by couplingthe NAND of the DOWN signal and the LOCK signal in place of the UPsignal in FIG. 3.

In the embodiment of FIG. 3, the inverters between the LOCK signal andthe NOR gate 5 and between the reset signal and the clear input of theflip-flop 3 are provided for buffering purposes. Other embodiments mayprovide alternative buffering or no buffering, as desired.

Moving now to FIG. 4, a schematic diagram of one embodiment of a delayelement is shown. Delay element 122 is a programmable delay element, andmay be used in programmable delay line 16 of FIG. 1, as well as beingused as one of the delay elements 122A-122F in phase detector 12. Delayelement 122 includes a plurality of capacitive delay elements. In theembodiment shown, field effect transistors (FETs) are used for thecapacitive elements. Alternate embodiments employing capacitive elementsother than FETs are possible and contemplated.

Delay element 122 is configured to receive a clock signal, such as thereference clock signal. The clock signal is input to the gate of a pairof transistors 124A and 124B forming an inverter. The inverter is loadedby one or more of the capacitive elements (transistors 130-136, throughtransistors 146-152, respectively) depending on the counter value. Theoutput of the inverter formed by transistors 124A and 124B is coupled tothe input of an inverter formed by transistors 126A and 126B, which isalso loaded by capacitive elements (transistors 138-144 throughtransistors 154-160, respectively) depending on the counter value.

Delay element 122 is a programmable delay element, wherein the amount ofcapacitance (and thus the delay) provided is binary weighted. In otherwords, the amount of capacitance provided by each capacitive element maybe related to the amount of capacitance of other elements by a binaryfactor (2, 4, 8, 16, etc.) For example, transistor 132 may have twicethe capacitive load as transistor 130, transistor 134 may have 4 timesthe capacitive load as transistor 130, and transistor 136 may have 8times the capacitive load as transistor 130. A similar relationship mayhold for transistors 138-144. The amount of delay provided may becontrolled by binary inputs to the circuit. The inputs illustrated inthe drawing each bring a pair of capacitive elements into the circuitwhen an active high signal is present on the respective signal lines (byactivating the transistors 146-160 which have gate terminals coupled tothose signal lines, as shown in FIG. 4). The inputs to the circuit maybe received from counter 14 (e.g. Counter(0), the least significant bitof the counter, through Counter(3) in the illustrated embodiment). Thus,(if C is the capacitive load of transistor 130 and of transistor 138),when the binary value of counter 14 output is 0001, a capacitive load ofC is added to the outputs of the invertors formed from transistors124A-124B and 126A-126B. When the binary value of to counter 14 outputis 0010, the capacitive load is 2C, when the binary value is 0011, thecapacitive load is 3C, and so on.

In other implementations, such as with the delay elements 122 of phasedetector 12, various ones of the binary signal inputs may be hardwiredto power or ground, thereby presetting the amount of delay provided to apredetermined value. It should also be noted that delay lines, eitherprogrammable or hardwired, may be formed by coupling multiple instancesof delay element 122 together.

Turning next to FIG. 5, a block diagram of a carrier medium 300including a database representative of DLL circuit 10 is shown.Generally speaking, a carrier medium may include storage media such asmagnetic or optical media, e.g., disk or CD-ROM, volatile ornon-volatile memory media such as RAM (e.g. SDRAM, RDRAM, SRAM, etc.),ROM, etc., as well as transmission media or signals such as electrical,electromagnetic, or digital signals, conveyed via a communication mediumsuch as a network and/or a wireless link.

Generally, the database of DLL circuit 10 carried on carrier medium 300may be a database which can be read by a program and used, directly orindirectly, to fabricate the hardware comprising DLL circuit 10. Forexample, the database may be a behavioral-level description orresister-transfer level (RTL) description of the hardware functionalityin a high level design language (HDL) such as Verilog or VHDL. Thedescription may be read by a synthesis tool which may synthesize thedescription to produce a netlist comprising a list of gates from asynthesis library. The netlist comprises a set of gates which alsorepresent the functionality of the hardware comprising DLL circuit 10.The netlist may then be placed and routed to produce a data setdescribing geometric shapes to be applied to masks. The masks may thenbe used in various semiconductor fabrication steps to produce asemiconductor circuit or circuits corresponding to DLL circuit 10.Alternatively, the database on carrier medium 300 may be the netlist(with or without the synthesis library) or the data set, as desired.

While carrier medium 300 carries a representation of DLL circuit 10,other embodiments may carry a representation of any portion of DLLcircuit 10, as desired.

While the present invention has been described with reference toparticular embodiments, it will be understood that the embodiments areillustrative and that the invention scope is not so limited. Anyvariations, modifications, additions, and improvements to theembodiments described are possible. These variations, modifications,additions, and improvements may fall within the scope of the inventionsas detailed within the following claims.

What is claimed is:
 1. A delay locked loop (DLL) circuit comprising: adelay line configured to delay the transitions of a reference clocksignal; a counter coupled to the delay line, wherein the counter iscoupled to provide a count to the delay line, wherein the amount ofdelay provided by the delay line is determined by the count; a phasedetector coupled to the delay line and a counter control circuit,wherein the phase detector is configured to receive an output clocksignal from the delay line and the reference clock signal, and whereinthe phase detector is configured to produce one or more output signalsdependent upon the phase relationship between the reference clock signaland the output clock signal; and the counter control circuit coupled toreceive the one or more output signals from the phase detector, whereinthe counter control circuit comprises a flip-flop, and wherein thecounter control circuit is configured, responsive to a reset of the DLLcircuit, to establish a first state in the flip-flop in which thecounter control circuit is configured to control operation of thecounter independent of the one or more output signals of the phasedetector, and wherein the flip-flop is clocked by a signal derived fromat least one of the one or more output signals of the phase detector,the flip-flop changing to a second state responsive to the signal, andwherein the counter control circuit is configured, responsive to thesecond state, to terminate the control that is independent of the one ormore output signals from the phase detector.
 2. The DLL circuit asrecited in claim 1 wherein the predetermined control comprises causingthe counter to increment, even in an event that the one or more outputsignals do not indicate increment.
 3. The DLL circuit as recited inclaim as recited in claim 2, wherein the predetermined state of the oneor more output signals includes one or more output signals transitioningfrom indicating increment to indicating decrement.
 4. The DLL circuit asrecited in claim 2, wherein the counter control circuit is furtherconfigured to generate a first signal that causes the counter toincrement and a second signal which causes the counter to decrement, andwherein the first signal is activated responsive to said reset andremains active until detection of the predetermined state.
 5. The DLLcircuit as recited in claim 1, wherein the DLL circuit is configured tolock at N=1, wherein N is an integer multiple of the period of thereference clock signal.
 6. The DLL circuit as recited in claim 1,wherein the phase detector and the delay line each include one or moredelay elements.
 7. The DLL circuit as recited in claim 1, wherein thecounter control circuit is configured to receive a reset signalindicative of the reset.
 8. The DLL circuit as recited in claim 7,wherein the counter is configured to receive the reset signal andfurther configured to reset to zero responsive to receiving the resetsignal.
 9. The DLL circuit as recited in claim 8, wherein the countercontrol circuit is configured to cause the counter to incrementresponsive to activation of the reset signal.
 10. The DLL circuit asrecited in claim 1, wherein the one or more output signals produced bythe phase detector include an UP signal and LOCK signal, wherein the UPsignal, wherein active, indicated that the counter is to increment, andwherein the LOCK signal, when active indicates that the DLL circuit hasachieved a phase lock.
 11. A method of operating a delay locked loop(DLL) circuit, the method comprising: generating one or more outputsignals from a phase detector, the one or more output signals indicativeof a phase relationship between a reference clock signal and an outputclock signal; establishing a first state in a flip-flop responsive to areset of the DLL circuit; controlling a counter independent of the oneor more output signals from the phase detector in response to the firststate; and clocking the flip-flop with a signal derived from at leastone of the one or more output signals of the phase detector, theflip-flop changing to a second state responsive to the signal, andwherein the counter control circuit is configured to terminate saidcontrolling the counter responsive to detecting said second state. 12.The method as recited in claim 11, wherein said controlling comprisescausing the counter to increment, even in an event that the one or moreoutput signals do not indicate increment.
 13. The method as recited inclaim 12, wherein said detecting comprises detecting a transition of theone or more output signals from indicating increment to indicatingdecrement.
 14. The method as recited in claim 13, wherein saidcontrolling comprises generating a first signal that causes the counterto increment and a second signal which causes the counter to decrement,and wherein the first signal is activated responsive to the reset andremains active until the transition.
 15. The method as recited in claim11 further comprising initializing the counter to zero responsive to thereset.
 16. The method as recited in claim 11, wherein the one or moreoutput signals produced by the phase detector include an UP signal and aLOCK signal, wherein the UP signal, when active, indicates that thecounter is to increment, and wherein the LOCK signal, when active,indicates that the DLL circuit has achieved a phase lock.
 17. A carriermedium comprising one or more databases representing: a delay lineconfigured to delay the transitions of a reference clock signal; acounter coupled to the delay line, wherein the counter is coupled toprovide a count to the delay line, wherein the amount of delay providedby the delay line is determined by the count; a phase detector coupledto the delay line and a counter control circuit, wherein the phasedetector is configured to receive an output clock signal from the delayline and the reference clock signal, and wherein the phase detector isconfigured to produce one or more output signals dependent upon thephase relationship between the reference clock signal and the outputclock signal; and the counter control circuit coupled to receive the oneor more output signals from the phase detector, wherein the countercontrol circuit comprises a flip-flop, and wherein the counter controlcircuit is configured, responsive to a reset of the DLL circuit, toestablish a first state in the flip-flop in which the counter controlcircuit is configured to control operation of the counter independent ofthe one or more output signals of the phase detector, and wherein theflip-flop is clocked by a signal derived from at least one of the one ormore output signals of the phase detector, the flip-flop changing to asecond state responsive to the signal, and wherein the counter controlcircuit is configured, responsive to the second state, to terminate thecontrol that is independent of the one or more output signals from thephase detector.
 18. The carrier medium as recited in claim 17, whereincausing the counter to count in a predetermined direction comprisescausing the counter to increment, even in an event that the one or moreoutput signals do not indicate increment.
 19. The carrier medium asrecited in claim 18, wherein the predetermined state of the one or moreoutput signals includes the one or more output signals transitioningfrom indicating increment to indicating decrement.
 20. The carriermedium as recited in claim 19, wherein the counter control circuit isfurther configured to generate a first signal that causes the counter toincrement and a second signal which causes the counter to decrement, andwherein the first signal is activated responsive to said reset andremains active until detection of the predetermined state.